1. Field of the Invention
The present invention relates to an improved stackable transport container, and in particular to an intermediate-bulk stackable re-useable composite container which meets the United Nations Standard Chapter 16 Packing Group II Standards for Composite Intermediate-Bulk Transport Containers.
Containers in this category are designed for the transport of liquids which may be toxic and/or corrosive and/or oxidisers (i.e. Class 5 and Class 8 liquids). Such containers must be corrosion-resistant, and must also be very strong, since typically each container will hold 1000 liters of liquid having a specific gravity of up to 1.8.
For economical use, the containers must be easy to handle, and for economical transport and storage, the containers must be stackable.
Containers of this type usually are required by users to be stackable 2- high.
2. Description of the Prior Art
To meet the criteria outlined above, a container must be corrosion-resistant and have the strength to withstand not only the pressures of the liquid contained in the container, but also the pressures imposed on the container when it is handled and stacked.
For commercial reasons, the containers must also be such that they can be made at an acceptable price, and have an acceptably long working life typically in excess of five years. United Nations regulations require that such containers are tested every five years.
To achieve the necessary corrosion-resistance, the best materials to use are stainless steel or a suitable known corrosion-resistant plastics material such as polyethylene or polyvinyl chloride, However, stainless steel is expensive as a material and also is expensive to fabricate. Plastics materials are less expensive and can be fabricated using cheaper methods (e.g. moulding) but hitherto have been unable to achieve the necessary strength without excessive wall thickness. If very thick walls are used, the cost of materials increases and the container itself is disproportionately bulky compared to the amount of liquid it contains.
It will be appreciated that any of a large range of materials and designs can be used for a suitable container if commercial considerations are ignored, but the requirements of an acceptable price and of achieving the necessary strength without excessive wall thickness impose difficult design and material criteria.
The shape of the container further increases the difficulties of design: it is well known that, for a given wall thickness, a circular cross-section container is very much stronger than any cross-sectional shape which has corners or bends, because no matter what the method of fabrication, bends or corners inevitably introduce areas of greater stress in the container walls at and adjacent the bends or corners. These stressed areas are weaker under load, so to compensate for this weakness, the overall thickness of the wall structure must be increased. This increases the bulk of the container, and its cost of manufacture.
Thus, a container having a cross-section which is square, rectangular, octangular, hexagonal or other readily-stackable shape, requires a very much greater wall-thickness than a container of circular cross-section, where no areas of raised stress occur, and the loading on the wall imposed by the fluid in the container is evenly distributed around the wall of the container. Thus, a circular-cross-section container is comparatively economical in materials. However, circular cross-section containers are awkward to handle and cannot be stacked to form a `solid` load i.e. with adjacent containers touching each other on all sides.
A number of different solutions to the above-described problems have been proposed in the prior art. For example, U.S. Pat. No. 5,156,268 (Nichols) discloses a rigid metal outer container of rectangular cross-section lined with a thin wall plastics inner tank, the walls of which are supported by the outer container. The rectangular cross-section gives good handling and stacking characteristics to the container, and the inner plastics container is corrosion-resistant so that the container can be used for corrosive liquids. The use of a metal outer container means that the container is of adequate strength to withstand both handling and stacking stresses and the stress of the contained liquid. However, the construction is far from ideal, since it is expensive to manufacture, and the metal outer container is susceptible to corrosion damage from spills from the inner liner.
U.S. Pat. No. 4,982,867 (Dubois) discloses a two part drum-like container in which the inner container is made of plastics material and the outer container is made of a corrugated fibre material. Both inner and outer containers are square in cross-section, but the sides of the square are slightly rounded and the corners are rounded; this is said to improve the structural strength of the container. The inner container is provided with one or more filling/emptying apertures in its upper surface but these apertures are non- accessible from the exterior of the container: the outer container fits over the inner container as a shell, and at least the top of the outer container must be removed for access to the inner container.
This container essentially is a compromise between a rectangular cross-section container for goods stacking and handling characteristics and a cylindrical container for strength. Unfortunately, like many compromises, the container essentially has the drawbacks of both types:- it is lower in strength than a cylindrical container, but is not as readily stackable or as easy to handle as a square or rectangular cross-section container because of the curvature of the sides. A further drawback is the lack of filling/emptying access without partly dismantling the container.
U.S. Pat. No. 4,595,112 (Dubois) relates to an insulated container in three parts: an inner liner, an insulating shell and a fibre drum. All three components are circular in cross-section and the container is essentially designed as an insulating container rather than for ease of transport.
U.S. Pat. No. 3,406,855 (McKechnie) discloses a two-part container which consists of a stainless steel inner tank generally rectangular in shape, with a fibre-glass reinforced plastic outer casing. The loss of strength due to the rectangular shape of the inner tank is compensated for by the fact that the inner tank is of stainless steel, but this has the drawback (discussed above) of greatly increased cost.
U.S. Pat. No. 5,259,509 (Boal) discloses a stackable storage container which has a pallet base carrying a cylindrical tank. The tank itself is a single skin construction and is separate from the pallet base. This design has the advantages of strength from the cylindrical shape of the tank, and the top of the tank is designed to cooperate with the pallet base so that the tanks can be stacked. However, this construction nevertheless has the drawback of being an awkward shape to handle and stack, and is also a very expensive type of construction.
French Patent No. 2134729 relates to plastics packaging for glass bottles, and discloses a two part protective shell for a glass or other fragile bottle. The lower half of the shell is provided with a hollow in which the glass bottle nests, the top half of the shell is a press or a screw fit over the bottom half of the outer shell. Both the bottle and the protective shell are of circular cross-section. The shell is simply a protective case for transport and is not permanently secured to the inner container in any way. Further, the outer shape of the completed unit is of circular cross-section, and therefore is difficult to handle or stack.
U.S. Pat. No. 5,232,120 (Dunken) discloses a single skin vessel which is circular in cross-section and frustro-conical in shape. The container may be moulded with an integral pallet base, or mounted on a pallet or seated in a protective tub which protects the base of the container and has the same outer shape as the base of the container. Thus, a container has the strength which results from a circular cross-section container, but is an inconvenient shape for handling or stacking.
U.S. Pat. No. 5,024,346 (Roser) discloses a storage and transport container which consists of a rectangular cross-section framework of metal bars which form an outer cage containing a flexible inner liner having the same general shape as the outer cage. The inner liner has no strength of its own:- all the load of the contained fluent material is taken by the outer cage.
U.S. Pat. No. 4,286,723 (Schutz) discloses a circular cross-section steel barrel with a blow moulded inner plastics liner, also of circular cross-section. The liner is non-load bearing and functions purely as a liner. The composite container is generally cylindrical in shape, and therefore is not easy to either handle or stack.
U.S. Pat. No. 3,782,602 (Page) discloses a single skin container of generally rectangular cross-section but with a slightly tapered shape so that the containers may be stacked one upon the other, for holding frozen or melted water or other liquid for domestic purposes. The container is not designed for commercial scale transport of toxic or corrosive liquids and is a single skin structure only.
U.S. Pat. No. 4,421,243 (Croley) discloses a fibre board disposable container which is supported upon a square pallet and holds a flexible bag liner which is non-load bearing. The container is generally cylindrical in shape and although the pallets base makes the containers stackable, the composite container/pallet never-the-less is an awkward shape for handling or stacking and occupies excessive space when stacked because of the overhanging pallet base.
It is an object of the present invention, to provide an intermediate bulk stackable composite container which overcomes the drawbacks of the prior art and provides a robust low cost composite container which has good handling and stacking characteristics but which can be manufactured relatively inexpensively from plastics material.
The use of a corrosion-resistant plastics material for such a container clearly is advantageous but hitherto the use of an all-plastics construction has been restricted by strength considerations:- a rectangular cross-section plastics container is simply not strong enough to withstand both the pressure of the contained liquid and the stacking pressures, without using an excessive wall thickness and making the container difficult to manufacture and uneconomic to use, and a cylindrical container, whilst strong enough to withstand the pressures upon it, is an inconvenient shape to stack and to handle.